Plural-wavelength flame detector that discriminates between direct and reflected radiation

• US 5,625,342 A
• Filed: 11/06/1995
• Issued: 04/29/1997
• Est. Priority Date: 11/06/1995
• Status: Expired due to Fees

• Current Assignee: Administrator of The National Aeronautics and Space Administration, US Government As Represented By The Director National Security Agency
• Original Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Inventors: Hall, Gregory H., Medelius, Pedro J., Barnes, Heidi L., Simpson, Howard J., Smith, Harvey S.
• Primary Examiner(s): Hofsass, Jeffery
• Assistant Examiner(s): Lee, Benjamin C.
• Application Number:
  US08/561,202
• Time in Patent Office: 540 Days
• Field of Search: 340/577, 340/578, 340/600, 137/65, 431/13, 250/554, 250/336.1, 250/339.01, 250/339.02, 250/339.05, 250/339.08, 250/339.15, 250/340, 250/372
• US Class Current: 340/578
• CPC Class Codes: F23N 2229/14 using two or more different types of flame sensor
• CPC Class Codes: F23N 2229/16 using two or more of the same types of flame sensor
• CPC Class Codes: F23N 5/082 using electronic means
• CPC Class Codes: G08B 17/12 Actuation by presence of radiation or particles, e.g. of infrared radiation or of ions
• CPC Class Codes: G08B 29/183 Single detectors using dual technologies G08B13/1672, G08B13/2448, G08B13/2494 take precedence

Abstract

A flame detector employs a plurality of wavelength selective radiation detectors and a digital signal processor programmed to analyze each of the detector signals, and determine whether radiation is received directly from a small flame source that warrants generation of an alarm. The processor'"'"'s algorithm employs a normalized cross-correlation analysis of the detector signals to discriminate between radiation received directly from a flame and radiation received from a reflection of a flame to insure that reflections will not trigger an alarm. In addition, the algorithm employs a Fast Fourier Transform (FFT) frequency spectrum analysis of one of the detector signals to discriminate between flames of different sizes. In a specific application, the detector incorporates two infrared (IR) detectors and one ultraviolet (UV) detector for discriminating between a directly sensed small hydrogen flame, and reflections from a large hydrogen flame. The signals generated by each of the detectors are sampled and digitized for analysis by the digital signal processor, preferably 250 times a second. A sliding time window of approximately 30 seconds of detector data is created using FIFO memories.

Claims

1. A flame detector apparatus comprising:

a) first and second radiation responsive detectors, said first radiation responsive detector being responsive to radiation of a first wavelength and said second radiation responsive detector being responsive to radiation of a second wavelength, wherein said first and second wavelengths are present in radiation from a flame to be detected, but are reflected differently off of a reflective surface, each said detector generating a signal in response to detected radiation;

b) processor means for analyzing the signals generated by said first and second detectors, and determining therefrom whether radiation detected by said first and second detectors is received directly from a flame, or is received from a reflection of a flame, said processor means including means for generating an alarm signal if it is determined that radiation detected by said first and second detectors is received directly from a flame; and

c) alarm indicator means responsive to said alarm signal for indicating that a flame has been detected.

2. The flame detector apparatus of claim 1, wherein said processor means further includes memory means for storing a sliding time interval window of data corresponding to the signals generated by said first and second detectors.

3. The flame detector apparatus of claim 1, further comprising means for testing the operation of said flame detector, said means for testing being controllable by said processor means and including at least one radiation source positioned to be detectable by said first and second detectors, said radiation source generating radiation including wavelengths to which said first or second detectors is responsive.

4. The flame detector apparatus of claim 1, wherein said processor means further includes means for determining whether detected radiation is generated by a small flame or a large flame.

5. The flame detector apparatus of claim 4, wherein said means for determining whether detected radiation is generated by a small flame or a large flame further comprises means for calculating a frequency spectrum of at least one of said detector signals, and determining a ratio of low frequency energy to high frequency energy in said frequency spectrum to determine whether said at least one of said detector signals is indicative of a small flame flicker frequency or a large flame flicker frequency.

6. The flame detector apparatus of claim 5, wherein said means for determining whether detected radiation is generated by a small flame or a large flame further comprises means for high pass filtering the frequency spectrum of said at least one of said detector signals to form a filtered spectrum if the determined ratio of low frequency energy to high frequency energy is above a threshold ratio, and then determining the ratio of low frequency energy to high frequency energy of the filtered spectrum to determine whether said at least one of said detector signals is indicative of a small flame flicker frequency.

7. The flame detector apparatus of claim 1, wherein said first radiation responsive detector is responsive to radiation of a first infrared wavelength, and said second radiation responsive detector is responsive to ultraviolet wavelength radiation.

8. The flame detector apparatus of claim 7, further comprising a third radiation responsive detector, said third radiation responsive detector being responsive to radiation of a second infrared wavelength.

9. The flame detector apparatus of claim 8, wherein said first radiation responsive detector is responsive to infrared radiation of approximately 2.7 micron wavelength, said second radiation responsive detector is responsive to ultraviolet radiation of wavelengths less than 300 nanometers, and said third radiation responsive detector is responsive to infrared radiation of approximately 1.3 micron wavelength.

10. The flame detector apparatus of claim 1, wherein said processor means further includes means for calculating the normalized cross-correlation of the signals generated by said first and second radiation responsive detectors to determine whether radiation detected by said detectors is received directly from a flame, or is received from a reflection of a flame.

11. The flame detector apparatus of claim 2, wherein said processor means further includes means for determining whether detected radiation is generated by a small flame or a large flame.

12. The flame detector apparatus of claim 3, wherein said means for determining whether detected radiation is generated by a small flame or a large flame further comprises means for calculating a frequency spectrum of at least one of said detector signals, and determining a ratio of low frequency energy to high frequency energy in said frequency spectrum to determine whether said at least one of said detector signals is indicative of a small flame flicker frequency or a large flame flicker frequency.

13. The flame detector apparatus of claim 4, wherein said means for determining whether detected radiation is generated by a small flame or a large flame further comprises means for high pass filtering the frequency spectrum of said at least one of said detector signals to form a filtered spectrum if the determined ratio of low frequency energy to high frequency energy is above a threshold ratio, and then determining the ratio of low frequency energy to high frequency energy of the filtered spectrum to determine whether said at least one of said detector signals is indicative of a small flame flicker frequency.

14. A flame detector apparatus comprising:

a) a first infrared detector being responsive to radiation of a first infrared wavelength and generating a first analog electrical signal in response thereto;

b) an ultraviolet radiation detector being responsive to ultraviolet wavelength radiation and generating a second analog electrical signal in response thereto;

c) an A/D converter for periodically sampling said first and second analog electrical signals, and converting them to first and second digital sample values, respectively;

d) a digital signal processor for receiving said first and second digital sample values, and calculating the normalized cross-correlation of said first and second digital sample values to determine whether radiation detected by said first and second detectors is received directly from a flame or from a reflection of a flame, said digital signal processor including means for generating an alarm signal if the calculated normalized cross-correlation exceeds a threshold value which indicates that radiation detected by said first and second detectors is received directly from a flame; and

e) alarm indicator means responsive to said alarm signal for indicating that a flame has been detected.

15. The flame detector apparatus of claim 14, wherein said digital signal processor is further programmed to calculate the frequency spectrum of at least one of said digital sample values, and determine from said frequency spectrum, whether radiation detected by said first and second detectors has a relatively high flicker frequency indicative of a small flame, or has a relatively low flicker frequency indicative of a large flame.

16. The flame detector of claim 14, further comprising:

f) a second infrared radiation detector responsive to radiation of a second infrared frequency and generating a third analog electrical signal in response thereto, said third analog electrical signal also being sampled and converted by said A/D converter;

, to form a third digital sample value;

wherein, said digital signal processor is programmed to calculate the normalized cross-correlation of each pair of digital sample values, and generate an alarm signal if any of said normalized cross-correlations exceeds said threshold value.

17. A method for detecting a flame which discriminates between a directly detected flame, and the reflection of a flame, said method comprising:

a) selecting a plurality of radiation wavelengths which are all present in a flame to be detected, but which have different reflection characteristics;

b) providing a plurality of wavelength selective radiation detectors, each said detector being responsive to radiation of a corresponding one of said plurality of wavelengths, and generating an electrical signal in response thereto;

c) positioning said plurality of detectors to receive radiation from an area in which a flame is to be detected;

d) calculating the normalized cross-correlation of each pair of detector signals;

e) comparing the value of said calculated normalized cross-correlation of each pair of detector signals to a threshold value above which indicates that the radiation detected by said plurality of detectors is received directly from a flame, and not from a reflection of a flame; and

f) generating an alarm indication if the normalized cross-correlation value of at least one pair of detector signals exceeds said threshold value.

18. The method of claim 17, wherein the steps of selecting, providing and positioning further comprise:

a) selecting first and second infrared radiation wavelengths and an ultraviolet radiation wavelength which are all present in a flame to be detected, but which have different reflection characteristics;

b) providing first, second and third wavelength selective radiation detectors, said first detector being responsive to radiation of said first infrared wavelength and generating a first electrical signal in response thereto, said second detector being responsive to radiation of said second infrared wavelength and generating a second electrical signal in response thereto, and said third detector being responsive to radiation of said ultraviolet wavelength and generating a third electrical signal in response thereto; and

c) positioning said first, second and third detectors to receive radiation from an area in which a flame is to be detected.

19. The method of claim 17, wherein the step of generating an alarm indication further comprises:

1) calculating a frequency spectrum of at least one of said detector signals;

2) calculating the ratio of low frequency components to high frequency components in said frequency spectrum; and

3) generating an alarm signal only if the ratio of low frequency components to high frequency components is below a preset threshold ratio value which indicates that the flicker frequency of the detected radiation is characteristic of a small flame.

20. The method of claim 18, wherein the step of generating an alarm signal further comprises:

i) high pass filtering the frequency spectrum if the ratio of low frequency components to high frequency components is above said preset threshold ratio value to form a filtered frequency spectrum;

ii) calculating the ratio of low frequency components to high frequency components in said filtered frequency spectrum; and

iii) generating an alarm signal if the ratio of low frequency components to high frequency components in said filtered frequency spectrum is below said preset threshold ratio value.